[DUAL_BOOT]Dual booting an android phone with an extrrnal SD card

So here you come. To read and perform this tutorial, you obviously need a first hand experience on flashing a ROM and/or kernels. Otherwise this tutorial and my efforts to get you a device with two OSes running might end up giving you a bricked device. So, if you're hearing the terms "flashing" or 'kernels' for the first time and thinking it's kinda good food, then bro, just go and taste those first.
Something's to remind before we gonna dig deep into this tutorial->
1> Noone but you will be responsible for what you end up with.
2> The warranty of your device will be voided after this if it isn't already after rooting. For MI users, the good news is that you can reclaim it by just flashing the fastboot ROM for your device.
Enough lectures. Bro let's get to work.
This you'll be needing =>
1> One working Windows PC(because I doesn't know any replacement of booting.exe on any other OS. If you know, then let me).
2> A class 10 memory card ( I recommend 32GB for the spaces)
3> A custom ROM and kernel for your phone(the second os)
4> Any custom CWM based recovery installed.(since TWRP is most popular, I will demonstrate using it. You can use any other you want overall process will be the same)
5> ADB, fastboot and the device drivers (easily found in XDA)
PART 1: MODIFYING THE BOOT

At first, how does your device boots up? What are the partitions called /data and /system? The answer is quite simple. It's your kernel that points out the location from where the OS should be picked up. So for booting into the second OS we need some modifications to it at first.
Search and download bootimg.exe on XDA, I'll post a link later. Create two folders. Name them "Internal OS" and "External OS" respectively. Put the zip file of the OS you're currently using to the first one and the OS you're gonna use on the external storage to the second one. Rename the second OS to originalExternalOS.zip. Extract originalExternalOS.zip. Pick the boot.img file from the root of the extracted folder and move it to a new folder named "boot2". Extract the IMG using bootimg.exe. Navigate to the initrd folder and you will get a file named 'fstab".
Basically it's the file that tells the kernel which partition does the OS resides in.
Open the file in your favourite text editor.

Replace every instance of the first line with the second one:
/dev/block/bootdevice/by-name/system => /dev/block/mmcblk1p2
/dev/block/bootdevice/by-name/userdata => /dev/block/mmcblk1p3
/dev/block/bootdevice/by-name/cache => /dev/block/mmcblk1p4

Save the file without giving any extension to it. Repack it using the same tool. You'll have boot-new.img and boot-old.img. Rename boot-new.img to boot.img and replace the one in the root folder with this. Basically what we're doing here is replacing the old boot.img with the modified one.

For your knowledge, blocks are the partitions of any storage you have on your device. For example, your internal storage is partitioned to near about 30 different blocks each starting with prefix "mmcblk0p". We here just told the kernel to load the OS from the blocks mentioned. We'll be creating these blocks in the external SD card next.

PART 2: PARTITIONING THE SD CARD

Connect your device with the memory card inserted to your PC. If you haven't installed fastboot, ADB, and the drivers, do it now.

READ THE FOLLOWING CAREFULLY

Reboot the device to recovery mode. Type the commands in cmd:

Code:

---

adb  shell
parted
unit MB
print
quit
umount external_sd

---

Read and store the minimum and maximum capacity of your card. Since different cards will have different capacities I will point it as variable MIN_SIZE and MAX_SIZE. You'll need to calculate and put the values in the commands. Now type the following commands on cmd:

Code:

---

parted /dev/block/mmcblk1
rm 1

//START_BLOCK = MAX_SIZE - 5000

mkpartfs primary fat32 MIN_SIZE START_BLOCK

//SYS_START = START_BLOCK+1
//SYS_END = SYS_START + 1200

mkpartfs primary ext2 SYS_START SYS_END

//DATA_START = SYS_END+1

---

Download from this link